It has been well recognized throughout the aircraft industry that, especially in the actuation of primary flight control surfaces, a failure mode causing the surface to be jammed can have catastrophic consequences.
Some current designs have employed a shear pin to join two separate control systems whereby in a case that one system becomes jammed a pilot would, by applying extra force on controls, cause the shear pin to shear thus permitting the pilot to continue controlling the flight utilizing the other side.
Some other designs utilize a dual control system one each on pilot and copilot side whereby a disconnect strut or a torque tube disconnect between the two systems is used. For example, the torque tube disconnect is shown in U.S. Pat. No. 5,782,436, dated Jul. 21, 1998. According to this patent the disconnect may be activated by pulling (or pushing) on the control column only from a non-jammed side. However, it requires a large force to do it. The FAA regulation require that the control system be certified for ultimate pilot effort load from not less than 150 lbs to as much as 450 lbs. At least 75% of that load must be carried thru a disconnect system before the system is disconnected (Ref. FAR 25.397 and 25.399). In the worst-case scenario assume the first pilot is in command and his side of control surface is jammed. First, he tries to control the aircraft but soon realizes he cannot. Second, he suspects a control jam but does not know which side may be jammed. Third, he pulls hard on control column trying to activate a disconnect from his side but still nothing happens. Fourth, he calls on copilot for help. Fifth, copilot acting from un-jammed side has to apply a significant force on control column to activate the disconnect. Sixth, once a disconnect is achieved only the copilot would be able to continue flying the aircraft. He may also face a possibility of dynamic over-control because of a sudden release of force following the disconnect.
All these designs have a common problem requiring an extra pilot effort, a delay in controlling and possible over-control of the aircraft in a dynamic action following the pin shear or a disconnect. In a critical phase of flight these problems may cause unpredicted consequences. Another drawback of many such designs is that it is irreversible during the flight; once the shear pin is sheared or disconnect is achieved the system cannot be converted back to the original configuration until landing and then resetting the system.
In addressing the fail safe problem there are control systems which retain the control when one of the cables breaks. Some use spring mechanism to operate as a single cable control system. In this case the spring rotates the pulley in one direction and remaining cable operates against the spring in the other direction. Other cable control systems disconnect the failed system so that a redundant system may be operated. Both designs require much higher cable tension forces then normal to operate.